1. Field of the Invention
The present invention relates to an optical amplifier and an optical power monitor applicable to the optical amplifier.
2. Description of the Related Art
In recent years, an erbium doped fiber amplifier (EDFA) and other optical amplifiers applicable to an optical communication system or an optical network system have been put to practical use. An optical signal supplied to or output from an optical amplifier has a signal spectrum superimposed on a noise spectrum. For example, in the case that a plurality of repeaters each having an optical amplifier are provided on an optical fiber transmission line, the noise spectrum generated in each optical amplifier is accumulated to cause a problem that optical power relating to the signal spectrum cannot be precisely monitored, so a solution to cope with this problem is required.
Conventionally known is an optical amplifier including an optical amplifying medium and means for pumping the optical amplifying medium so that the optical amplifying medium has a gain band. For example, in an optical pumping type of optical amplifier having a doped fiber doped with a rare earth element as the optical amplifying medium, pump light having a properly set wavelength is supplied to the doped fiber. When an optical signal is input into the doped fiber being pumped by the pump light, the optical signal is amplified in accordance with the principle of stimulated emission. Further, in a semiconductor laser type of optical amplifier, a bias current is supplied to an optical amplifying medium provided as a semiconductor chip, thereby pumping the optical amplifying medium.
In a practical optical amplifier, optical power is monitored for various purposes. For example, in order to stop the pumping of the optical amplifying medium when an optical signal input into the optical amplifier is cut off, the power of the optical signal to be supplied to the optical amplifying medium is monitored. Further, in order to perform control for maintaining the output power of the optical amplifier constant, the power of the optical signal output from the optical amplifying medium is monitored.
To monitor the power of an optical signal, the optical signal is supplied to a photodiode, for example. When an optical signal is supplied to a photodiode being reversely biased, a photocurrent according to the power of the optical signal is generated in the photodiode, thereby monitoring the power of the optical signal. In the case of monitoring the power of an optical signal output from an optical amplifying medium, the optical signal has a signal spectrum superimposed on a noise spectrum generated in the optical amplifying medium. Therefore, the power of the optical signal relating to the signal spectrum cannot be precisely monitored only by opto/electrical conversion of the optical signal through the photodiode. Further, also in the case that noise spectra are accumulated by the use of a plurality of optical amplifiers cascaded, a similar problem arises.